In the metal industry several semi-products in various widths and thicknesses are produced by using different processes (e.g., hot and cold rolling, casting rolling etc.), these products being processed in the form of coils. Before and during processing, the coils are often subjected to heat treatment. The objects of heat treatment can be different, depending on the material qualities of the semi-product and on requirements relating to ultimate use and include annealing, tempering, stabilizing (stress-relieving), broaching etc.
Traditionally, heat treatment is performed in annealing furnaces. Three main processes are known. In the best-known process, the material charged into the furnace is subjected to heat treatment in a stationary state, i.e. it is not moved during the process. The second group of processes, use an arrangement in which coils move from one end of the relatively long furnace to the other end continuously or intermittently. In processes of the third type, the semi-product; in coils is continuously uncoiled. The common characteristic of the three types of processes lies in that heat treatment takes place by means of a heat carrier (air, gas, fluidized medium etc.).
The drawbacks of the traditional heat treating processes are well known. The construction and operation costs are rather high. The spatial requirements are also considerable. Due to indirect dissipation heat transfer, the degree of utilization of thermal energy is low and the duration of heat treatment is long. A further problem lies in the inhomogeneity of mechanical properties of the material subjected to heat treatment and the possibility of surfacial damage.
Heat treating processes are also known, in course of which the semi-product in a coil form is submitted to a heat treatment during rewinding in such a manner that over a given section electrical current is passed through the material, as described in the Italian Pat. No. 679,042 and in the British Pat. No. 1,200,089. Theoretically, with respect to the energy relationships, these processes are considered as advantageous, but owing to difficulties accompanying current input and due to complexity of control and possible damage to the material treated, the processes described above have not come in a general use in industry.
When strap coils are obtained from the strip mill, value of the insulation resistance to be measured between the single metal layers lying above each other, amounts to a considerable value. It has been found that insulation resistance can be considered as a product of the common insulating properties of the lubricant applied in course of rolling, of the oxide formed on the surface and the air gap, produced in course of coiling; as a consequence, in strap coils, as in roll-type foil condensers -- between the metal layers of the single turns an insulating layer is formed. In case if to the ends of said coil an electrical supply unit is connected, at a given voltage electrical current can be passed through the coil without producing a breakdown or short-circuit between the turn-to-turn insulations. The value of the so-called turn-voltage between the single turns represents a function of the resistance of the insulating layer.
If in dependence of the insulation resistance of the insulating layer, direct or alternating current with a low turn-voltage (e.g., 0.2 V) and a high current intensity (e.g. 1000 A) is led through the coil, the coil is heated by the heat generated by the current. Taking into consideration that the electrical resistance of the coils is relatively low, even with a low supply-voltage a heating current of high intensity can be led through the coil.
Between the heating capacity generated and the electrical resistance of the strap there is a linear relation and between the heating capacity and the intensity of the current the relation is quadratic; as a consequence, besides high current intensities, relatively short heat treating duration (e.g. 1-60 minutes) can be achieved.